This invention relates to sealing of cavities in high density polycrystalline ceramic bodies and, more particularly, to the sealing of high pressure discharge lamps composed of alumina, yttria and the like.
Electrical discharge devices, such as high pressure sodium vapor arc lamps, commonly utilize transparent or translucent high temperature refractory tubes composed of alumina. Within the alumina tube an electric arc extends between two tungsten electrodes to which current is conducted by a hermetically sealed feedthrough assembly. Because alumina and niobium metal have nearly equal thermal coefficients of expansion, a niobium tube or a niobium wire is used in high pressure sodium vapor lamps to conduct electrical current through the ends of the alumina arc tube. The joint between the niobium metal and the alumina is typically filled with a meltable frit based on calcium aluminate. Thus, the feedthrough assembly not only seals the discharge tube but also conducts electrical current through the end of the alumina arc tube.
While niobium is generally satisfactory as a closure member for alumina arc tubes, it is a relatively expensive metal and is in potentially short supply under certain world conditions. It is, therefore, desirable to provide a substitute for niobium in the sealing of high pressure arc discharge tubes.